Through Serial to Parallel conversion, a serial high bit rate channel is split into lower bit rate channels, in order to transmit them as parallel channels between a same transmission start point to a same transmission stop point, while the parallel channels do not have any crosstalk ideally. The parallel channels are then serialized into a high bit rate channel, which contains the transmitted information of the original serial channel. This makes it possible to decrease the effect of distortions and impairments appearing during the transmission, whose effects increase with the bit rate. It also makes it possible to reduce the spectrum width of transmitted signals because the parallel channels have a lower bit rate than the serial channel.
Known ways to implement parallel transmissions include: Polarization Multiplexing (PM), in which two signals are transmitted through the same medium by two signals having orthogonal polarizations; Wavelength Division Multiplexing (WDM), in which the parallel channels are transmitted through the same medium by different optical carriers having different wavelengths; Space Division Multiplexing (SDM), in which the parallel channels are transmitted through different mediums, which can be bounded such as a fiber ribbon; combinations of the previous multiplexing formats.
PM is recognized as an efficient way to double a transmitted data rate of an optical transmissions system as it makes it possible to carry two light signals on orthogonal polarizations of an optical fiber used as transmission medium. Both of the signals ideally do not interfere as the polarizations are orthogonal. Therefore, it is a way to improve the efficiency of the use of bandwidth of the optical fiber. As the bit rate of a total transmitted signal is doubled, whereas the baud rate of each polarization signal is unchanged, the width of the optical spectrum is unchanged. Therefore, PM does not degrade a tolerance to optical filtering occurring on the path of the light signal transmitted inside a network when compared with one single polarization, although a double of information is transmitted. However, polarization demultiplexing is required in order to recover each of the polarization multiplexed signals. One example of PM is given in “Precise Analysis of Transmission Impairments of Pol-Mux 110 Gb/s RZ-DQPSK with Automatic Pol-Dmux using Straight 2,000-km SMF Line” (ECOC 2008, paper We.1.E.6) by T. Ito and al.
One known method to implement PM is to provide two transmitters emitting the same wavelength and then to rotate the polarization of the signal emitted from one transmitter before combining it with the signal emitted from the other transmitter, so that both polarization are orthogonal to each other. Typically, the optical transmitters used in optical transmission systems are provided with a laser source emitting a continuous wave optical carrier signal and an optical modulator. The light carrier signal emitted from a laser source is linearly polarized; the laser source and the optical modulator are connected by a polarization maintaining fiber. Therefore, the light signal emitted from the transmitter is linearly polarized. In this configuration, rotating the polarization in one transmitter can be performed by a polarization controller or simply by rotating one axis of the polarization maintaining fiber carrying the light signal. A light signal resulting from a combination of the light signals emitted from two transmitters in this configuration is polarization multiplexed.
An alternative implementation method consists of using a same light source for both of the polarizations. A polarization maintaining coupler can be used to split a light carrier signal emitted from the laser source and each of the signals split by the coupler can be fed to an optical modulator. The polarization rotating scheme and combining scheme are identical to the above-mentioned implementation.
A binary data stream to be transmitted with the light signal may be pre-coded and de-serialized into tributary binary data streams; each of the tributary streams may be allocated to one driver, which generates a voltage used to drive an optical modulator imprinting the information to the light signal of one of the multiplexed polarizations. The multiplexed polarizations may be randomly and dynamically rotated while the light signal is transmitted through the fiber, although they remain orthogonal. Therefore, a dynamic polarization de-multiplexing scheme is useful on the receiver side.
One implementation of polarization demultiplexing can be qualified of optical polarization demultiplexing. It consists of rotating both of the multiplexed polarizations by a polarization controller so that they can be split by a polarization beam splitter or another polarization separating device. The polarization controller can be actively controlled so that the beam splitter correctly separate the polarization multiplexed signals, even when the polarizations are rotated while being transmitted through the optical fiber. Each of the separated polarizations is received by a separate optical receiver, which decodes the data of the light signal and converts it into a binary data stream. Electrical signals resulting from both of receivers are then serialized and the resultant binary data stream may be decoded. This is described in “Comparison of 100 Gb/s transmission performances between RZ-DQPSK and polarization multiplexed NRZ/RZ-DPSK with automatic polarization de-multiplexer” (OFC 2008, paper JThA46) by T. Ito and al.
An alternative method of performing polarization demultiplexing can be qualified of digital polarization demultiplexing. A light signal is received by a coherent receiver. The resultant lightwave signals can be converted into digital signals by analog to digital converters (ADCs). The resultant digital data can be calculated in accordance with appropriate algorithms that can retrieve and separate the data imprinted on each multiplexed polarization. The retrieved data streams are then decoded and serialized. One example is given in “PLL-Free Synchronous QPSK Polarization Multiplex/Diversity Receiver Concept with Digital I&Q Baseband Processing” (Photonics Technology Letters, Vol. 17, No. 4, April 2005) by R. Noe.
The binary data stream before serial-to-parallel conversion, emission and transmission through polarization multiplexed light signals and the binary data stream after polarization demultiplexing, reception and parallel-to-serial conversion are ideally identical. Differences between the streams are transmission errors. One data bit will be de-serialized and then allocated to the optical modulator imprinting data on the light signal of one polarization. This data will travel electrically from the de-serializer to the optical modulator, and then it will be transmitted optically from the modulator to an optical polarization combiner. This bit will be transmitted through a medium and received by a receiver. Another data bit of the serial data stream will be de-serialized in the same way but it will be allocated to the optical modulator imprinting data on the light signal of the other polarization. This other bit of data will travel electrically from the de-serializer to the other optical modulator, and then it will be transmitted optically from the other modulator to the optical polarization combiner. This other bit will be transmitted through the medium and received by a receiver. Therefore, both of the considered bits will travel on different electrical paths from the de-serializer to their optical modulators and on different optical paths from their modulators to the polarization combiner.
Differences in electrical or optical path lengths for the considered bits results in time difference for the transmission on the multiplexed polarizations. This problem is known as intra-channel skew. If the skew is more than half a symbol period of the signal transmitted on one polarization, the bits travelling through the longer path will arrive to the serializer in the receiver with a delay of more than one bit when compared to the bits travelling through the shorter path. This situation results in a change of the bit sequence during the transmission, i.e. in a transmission error.
Assuming 100 Gbit/s Dual Polarization Quadrature Phase Shift Keying (QPSK) transmission, the bit rate of the serial binary data stream is 100 Gbit/s but the symbol rate of the transmitted light signal is 25 Gbaud. Therefore, the symbol period of the light signal on one polarization is 40 ps. This corresponds roughly to the length tolerance of 4 mm of optical fiber for the optical path or 4 mm of wire for the electrical path. Considering the fact that the optical components for each polarization paths are usually connected by spliced fiber, such a tolerance may be hard to achieve on production. Moreover, Differential Group Delay (DGD) accumulated on the fiber during transmission may delay one polarization compared to the other one, and therefore randomly and dynamically change the intra-channel skew.
In addition, dynamic network reconfiguration or switching may cause a value of a link skew to change dynamically. If the skew is more than half a symbol period, the transmission is not error free with respect to the received data after parallel-to-serial conversion. Therefore, there is a need for monitoring the delay between polarization multiplexed signals in order to use this information to correct this delay and achieve correct transmission of data.
Various approaches have been proposed in order to solve this problem. The most obvious one is to use the Forward Error Correction (FEC) data of the serialized received data. This enables to check if the data is correct, in which case the delay between polarizations does not need to be corrected. If the error rate is more than 0.5 or in the order of 0.5, both polarizations are likely to be delayed and one can try to change this delay by using an optical delay, electrical delay or buffer on the path of one of the data transmitted on one of the multiplexed polarizations. Using the FEC information is possible in order to correct the delay between polarizations. In case of optical polarization demultiplexing, this can be done using an optical variable delay line on the path of the light signal of the polarization to be controlled, or an electrical phase shifter or a buffer on the path of the electrical signal of the polarization to be controlled.
In case of polarization demultiplexing through digital signal processing, it is possible to place a buffer or a filter to delay the data of the polarization to be controlled, as explained in “”Digital Communication Receivers” (Wiley-Interscience Publication, 1998, p 505) by H. Meyr et al. However, in all cases, the information given by the result on the FEC frame monitoring is binary in the sense that if a delay appears, there is no information on the delay amount or on which polarization is delayed.
Therefore, correcting the delay between polarization-multiplexed signals with this method is likely to require several attempts before finding the adequate delay compensation, complicating the practical implementation. Moreover, continuous control of the delay between polarized signals is not implementable with this method. Finally, using this method requires to use the information of upper layer of the network to control physical layer parameters, which may complicate system design or hinder interoperability.
Another approach is particular to digital polarization demultiplexing. In this case, a coherent receiver is used and the resulting signals are converted from analog to digital. The converted signals are processed in order to recover the transmitted data. During the signal processing, the signals for each polarization are recovered through algorithms such as Constant Module Algorithm (C.M.A.). This is explained by N. Kaneda et al. in “Coherent Polarization-Division-Multiplexed QPSK Receiver with Fractionally Spaced CMA for PMD Compensation” (Photonics Technology Letter, Vol. 21, No. 4, Feb. 15, 2009).
Digital processing enables to process the signal through digital filters, which enables to correct some of the degradations of the signal appearing during the transmission through the medium. Filters can be updated through the recognition of training sequences used at regular interval during the transmission, as described in “Ultra Long-Haul QPSK Transmission using a Digital Coherent Receiver” (LEOS 2007) by S. J. Savory et al. Training patterns can also be used to eliminate the ambiguity on polarizations and transmitted symbols. All possibilities for the attribution of ambiguous variable are tried, until the training pattern is recognized.
In this case, buffer or time delaying filters can be used to correct the delay between polarizations until the training pattern is recognized, or the received pattern can be compared with several recognition patterns, for possible cases of delay, until there is a match, which enables to retrieve and correct the delay between polarizations. This approach requires training patterns, which are susceptible to decrease the amount of transmitted data at constant bit rate. Moreover, this method requires one comparison for each case of delay. Therefore it multiplies the calculation time and required memory by the number of cases to study. This will consummate more electrical power for the increased computation requirements.
However, there is room for improvement in simplicity, possibility of continuous control, processing power consumption, monitoring range and speed for monitoring the delay between polarization multiplexed signals.
Parallel transmission can be implemented with WDM. The parallel lower rate channels are transmitted by lightwave carrier at different wavelengths through the same medium. Each channel is emitted by a transmitter and the light of all transmitters are multiplexed into the same fiber. At the other end of the link, the wavelengths are demultiplexed according to their wavelengths; each channel is received and decoded by receivers. The electrical data from the parallel receivers are then converted from parallel to serial. Intra channel skew can appear inside the medium due to chromatic dispersion or after reception due to difference of length of the electrical path. Dynamic network management and switching can change dynamically the intra-channel skew of received signals as the transmission path is changed dynamically. With higher bit rate, the symbol period decreases, therefore the skew problem becomes critical. This is illustrated in “Terabit LAN with optical virtual concatenation for Grid applications with super-computers” by M. Tomizawa et al. (OFC 2005 OThG6).
The presented solution rectify the skew between channels is the use of the XAUI standard, which relies on channel decoding and realigning. This requires an increase of the total bit rate for the same transmitted data payload, as the prefix needed for alignment is introduced in the transmitted data. In addition, the skew monitoring requires the information of upper layer information.
There is a room for improvement in term of simplicity, efficiency of the transmitted data rate.
Another way of implementing parallel transmission is SDM. The use of fiber ribbon for SDM and the skew problem inherent to SDM is illustrated in “All Optical Bit Parallel Transmission Systems” by A. P. Togneri et al. (SMBO IEEE 2003). The link skew is due to difference in the fiber length or condition. With higher bit rates, the symbol rate transmitted through the fiber increases, meaning shorter symbol periods. Therefore the skew problem becomes critical with higher bit rates. US Patent US2000484961A discloses a method to compensate skew by decoding and realigning data. This method requires the use of prefixes, which necessitates an increase of the total bit rate for the same transmitted data payload. Moreover, this method requires the use of information from higher layers than the physical layer.
There is room for improvement in term of simplicity, efficiency of the transmitted data rate. There is a need for a fast, simple method to monitor skew between polarization parallel channels on wide skew ranges, without relying on the information of upper layers, without the presence of a training sequence or prefix.
In conjunction with the above description, Japanese Patent Application Publications (JP-P2003-218844A, JP-P2004-193817A, and JP-A-Heisei 11-341102) enable to monitor and correct the skew between parallel channels.
Japanese Patent Application Publication (JP-P2003-218844A) requires that a special pattern named PING or PONG depending on the case is added to the actual data to be transmitted. PING and PONG pattern carry no information and are used only for the purpose of skew monitoring or compensation. Therefore, to carry effectively m bits of data within the network, JP-P2003-218844A requires in fact m bits plus the number of bits contained in PING or PONG to be physically transmitted through the network. Therefore, the bandwidth of the transmitters and receivers in the network when JP-P2003-218844A is implemented has to be higher than the bandwidth necessary to carry the data. This increases the cost and complexity of receivers and can cause a degradation of the performance of the receiver. Moreover, in JP-P2003-218844A, the receiver must distinguish PING and PONG patterns from the data. Therefore, PING and PONG patterns must be composed so they cannot be mistaken for data and data must be encoded so that it cannot be mistaken for PING or PONG pattern. In addition, measuring skew between channels in JP-P2003-218844A requires that the data received is first decoded so that PING and PONG patterns can be read and compared. When errors appear due to noise or degradation of the signal transmitted through the network, this can affect read PING and PONG patterns, and therefore cause an error in the skew evaluation. As a consequence, this can have a tremendous impact on the serialized signal that FEC or other correction method may not be able to compensate. Finally, in JP-P2003-218844A, the skew is measured in shifts of bits or symbols of PING and PONG patterns transmitted through the network. This is a coarse estimation which is limited to a one symbol period resolution.
Also, Japanese Patent Application Publication (JP-P2004-193817A) requires that special patterns named I(n), where n is an integer indexing the parallel channel where the pattern is inserted, are added to the actual data to be transmitted. I(n) patterns carry no information and are used only for the purpose of skew monitoring or compensation. Therefore, to carry effectively m bits of data within the network, JP-P2004-193817A requires in fact m bits plus the number of bits contained in I(n) patterns to be physically transmitted through the network. Therefore, the bandwidth of the transmitters and receivers in the network when 2004-193817 is implemented has to be higher than the bandwidth necessary to carry the data. This increases the cost and complexity of receivers and may cause a degradation of the performance of the receiver. Moreover, in JP-P2004-193817A, the receiver must distinguish I(n) patterns from the data. Therefore, I(n) patterns must be composed so they cannot be mistaken for data and data must be encoded so that it cannot be mistaken for I(n) patterns. In addition, measuring skew between channels in JP-P2004-193817A requires that the data received is first decoded so that I(n) patterns can be read and compared. In cases where errors appear due to noise or degradation of the signal transmitted through the network, this can affect the reading of I(n) patterns, and therefore cause an error in the skew evaluation. As a consequence, this can have a tremendous impact on the serialized signal that FEC or other correction method may not be able to compensate. Finally, in JP-P2004-193817A, the skew is measured in shifts of bits or symbols of I(n) patterns transmitted through the network. This is a coarse estimation which is limited to a one symbol period resolution.
Also, Japanese Patent Application Publication (JP-A-Heisei 11-341102) requires that m′ frame bits are added to the actual data to be transmitted. Therefore, to carry effectively m bits of data within the network, H11-341102 requires in fact m+m′ bits to be physically transmitted through the network. Therefore, the bandwidth of the transmitters and receivers in the network when JP-A-Heisei 11-341102 is implemented has to be higher than the bandwidth necessary to carry the data. This increases the cost and complexity of receivers and may cause a degradation of the performance of the receiver. Moreover, in JP-A-Heisei 11-341102, the receiver must distinguish m′ bits of the frame from the data. Therefore, the frame must be especially composed so it cannot be mistaken for data and data must be encoded so that it cannot be mistaken for the frame. Other proposed methods of generating the frame requires information from layers higher than the physical layer of the network in order to monitor and compensate skew to the channel. Such requirement increases the complexity of the receiver. Skew monitoring is dependant on higher layer information and in return, higher layer information necessitates skew compensation to be retrieved, and skew compensation depends on the monitored skew. This may have a negative impact on the stability of the integrity of the received data when distortions appear on the transmission path. Finally, in JP-A-Heisei 11-341102, the skew is measured in shifts of bits or symbols of the frame transmitted through the network. This is a coarse estimation which is limited to a one symbol period resolution.